The development of active corrosion protection systems for metallic substrates is an issue of prime importance for many industrial applications. The use of refined metals is widespread, but such metals can frequently be chemically reactive, limiting their use or requiring protective coatings. Protecting the surface of such metals has developed into a significant industry.
Corrosion of metals is a significant destructive process, resulting in huge economic losses, especially in the aerospace, automotive, and petroleum industries. An intense effort is underway to find coatings that inhibit the process of metal corrosion, a problem costing US industries more than $200 billion annually.
Conventional approaches to protecting metal surfaces can include organic coatings, paints, varnishes, polymer coatings, formation of oxide layers, anodisation, chemical modification, such as the formation of thiol and/or sulphate layers, and coating, for example via electroplating, with other metals or alloys. Such conventional approaches suffer from a variety of limitations, such as susceptibility to heat damage, necessity of thick coatings, cost, formation of waste products, etc. Thus, there is a need to address the aforementioned problems and other shortcomings associated with traditional metals and coatings.
Corrosion can be inhibited or controlled by introducing a stable protective layer made of inert metals, conductive polymers, or even thiol-based monolayers between a metal and a corrosive environment. However, these protective coatings have their limitations.
Thiolated self-assembled monolayers (SAMs) can only be assembled onto some metals (e.g., gold and copper) and do not withstand temperatures higher than ˜100° C. Polymeric coatings are relatively thick and may significantly change the physical properties of the underlying material. Polymer or sol-gel coatings are normally applied on the metal surface providing a barrier for permeation of corrosive species. However, when the barrier is damaged and the corrosive agents penetrate the metal surface the coating system cannot stop the corrosion process.
At present the most effective anti-corrosion coatings for active protection of metals are chromate-containing systems. However, this product is well-known to be toxic and carcinogenic and is now prohibited on end products and highly regulated on industrial lines by European Community directives (VUH and RoHS).
Therefore, there has been an extensive search for green and human safe anti-corrosion treatments. These smart coatings must be able to deal with local pH changes in the corrosive area but also be able to prevent interactions of corrosive species such as water, chlorine ions and/or oxygen, with the metal surface.
Graphene, a single atomic monolayer of graphite, has properties which are well suited to corrosion-inhibiting coatings in applications such as microelectronic components (e.g. interconnects, aircraft components, and implantable devices). Graphene is chemically inert, stable in ambient atmosphere up to 400° C., and can be grown on the meter-scale and mechanically transferred onto arbitrary surfaces. Single-layer graphene films and films consisting of a few layers of graphene have good transparency, so thin graphene coatings, for example having up to 4 layers of graphene, do not significantly affect the optical properties of the underlying metal.
Recently, Bunch et al. [Bunch, J. S.; Verbridge, S. S.; Alden, J. S.; van der Zande, A. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L Impermeable Atomic Membranes from Graphene Sheets, Nano Lett. 2008, 8, 2458-2462] have shown that single-atomic graphene films are impermeable to gas molecules. Chen et al. [Chen, S.; Brown, L.; Levendorf, M.; Cai, W.; Ju, S. Y.; Edgeworth, J.; Li, X.; Magnuson, C. W.; Velamkanni, A.; Piner, R. D. “Oxidation Resistance of Graphene-Coated Cu and Cu/Ni Alloy” ACS Nano 2011, 5, 1321-1327] have demonstrated that graphene can inhibit oxidation of the underlying copper metal. Prasai et al. [Prasai, D.; Tuberquia, J. C.; Harl, R. R.; Jennings J. K.; Bolotin, K. I.; “Graphene: Corrosion-Inhibiting Coating” ACS Nano, 2012, 6, 1102-1108] have demonstrated that graphene films can serve as corrosion-inhibiting coatings. US 2011/0151278 discloses magnetic devices/media having a graphene overcoat, while US 2010/0203340 is directed to protective carbon coatings, namely protective graphene coatings.
However, the existing graphene-coating technology has drawbacks. For example, the growth and/or transfer processes of graphene inevitably result in pinholes, defects or cracks in the film. This causes problems with corrosion, for example oxidative corrosion, at edges, defects and/or grain boundaries. For example, the metal substrate corrodes under the coating at points where there are pinholes in the graphene film. Efforts to solve these problems have so far concentrated on improvements in growth techniques and transfer techniques.
In addition, there is a need for protective coatings which can be thicker than a few layers while maintaining excellent transparency.
For applications in magnetic devices, there is a need to reduce the thickness of the current protective coatings to less than 1 nm thickness while maintaining the overall mechanical strength and its corrosion protection, in order to increase the magnetic recording density.
Silicon-doping has the potential to be used as an effective method for opening the band gap of graphene. Chemically silicon-doped graphene results in an energy gap as large as 2 eV according to density-functional theory (DFT) calculations. (Azadeh M. S. S.; Kokabi A.; Hosseini M.; Fardmanesh M.; “Opening in the Proposed Structure of Silicon Doped Graphene”, Micro Nano Lett. 2011, 582-585).
Modelling studies indicate that Si-doped graphene has the potential to detect or reduce harmful nitrogen oxides. While adsorption of the three nitrogen oxide molecules on pristine graphene is very weak, Si-doping enhances the interaction of these molecules with the graphene sheet (Chen Y.; Gao B.; Zhao J. X.; Cai Q. H.; Fu H. G; “Si-doped graphene: an ideal sensor for NO- or NO2-detection and metal-free catalyst for N2O-reduction” J. Mol. Model. 2012, 18, 2043-2054).
Si doping has also the potential to increase the photoluminescence of graphene (Lounis S. D.; Siegel D. A.; Broesler R.; Hwang C. G.; Haller E. E.; Lanzara A. “Resonant photoluminescent charging of epitaxial graphene”, Appl. Phys. Lett. 2010, 96, 151913).
The current invention provides a silicon-doped graphene coating with enhanced corrosion inhibition, transparency and/or gas barrier properties. The coatings of the current invention also exhibit self-healing properties. The formation of a passivation layer, which fills the pores present in the films leads to improvements in corrosion resistance. Doping with silicon also improves the transparency and therefore will allow the formation of thicker layers where protection with a transparent coating is important.
A further potential advantage of the Si doped graphene coating is an enhancement of the bio- and hemo-compatibility. This has not previously been demonstrated in graphene although an advantage has been observed for Si doped Diamond Like Carbon, DLC (Okpalugo T. I. T.; Ogwu A. A.; Maguire P. D.; McLaughlin J. A. D. “Platelet adhesion on silicon modified hydrogenated amorphous carbon films”. Biomaterials 2004; 25, 239.)
Silicon-doped graphene offers a unique combination of properties that are ideal for corrosion-inhibiting coating in applications such as microelectronic components. For example it could be used on the copper interconnects in computer chips, implantable medical devices, high-tech equipment (aerospace, super cars, and so on), or designer goods, where silicon-doped graphene's negligible size and weight, and improved transparency would be highly desirable.